This invention relates to a humidity sensing element which comprises a dielectric oxide layer formed by anodization of a valve metal body and a porous layer of semiconductive metal oxide with an electrode layer coated thereon and exhibits changes in its electrostatic capacitance with changes in humidity in the surrounding atmosphere, and a method of producing the same.
Measurement of humidity with high precision is still difficult compared with measurement of other physical variables of the atmosphere such as temperature and pressure. However, the need for easy and accurate measurement of humidity is increasing in many fields such as food industries, agriculture, air conditioning and medical service either to control humidity or to accommodate something to humidity.
As a general trend, preference has been given to electrical methods of measuring humidity. One of the now prevailing methods of this category is the use of a deliquescent salt, such as lithium chloride, which undergoes a change in its ionic conductivity with a change in its moisture content, and another is the use of a hygroscopic substance, such as magnetite or a silicon semiconductor, which exhibits a change in its electrical resistance as it adsorbs and desorbs moisture. However, humidity sensors embodying these methods, i.e. humidity sensors utilizing ionic conductivity, exhibit considerable drifiting of the indications with the passage of time by reason of polarization. Besides, electrical indications of these humidity sensors are influenced also by the adsorption of various gaseous substances other than moisture. Furthermore, these humidity sensors are not fully satisfactory in their responsiveness, mode of hysteresis and the width of humidity range they can cover.
Another category of conventional electric humidity sensors comprise a filament of an organic material such as human hair, nylon or polystyrene in combination with a strain gauge or microswitches so as to detect the deformation of the filament caused by adsorption and desorption of moisture. However, these humidity sensors are unsatisfactory in their accuracy, responsiveness, hysteresis and heat resistance. A still different type of humidity sensor utilizing swelling of a synthetic resin containing fine particles of an electronically conductive material such as carbon or a metal is low in sensitivity, unsatisfactory in responsiveness and weak to high temperature. A further example of known methods is the use of a porous alumina layer for detecting a change in humidity as a change either in electric capacitance or impedance of the alumina layer resulting from adsorption of moisture in the pores, or desorption therefrom. A drawback of this method is a considerable drifting of the dependence of the capacitance or the impedance on humidity with the passage of time.
Highly accurate measurement of humidity is possible by means of an apparatus using the principle of .alpha.-ray absorption and transmission, but such an apparatus is too large-scaled and too costly to be of general use.
Also there is an apparatus to convert the indications of a traditional wet and dry bulb hygrometer into electrical signals, but this apparatus, too, is large-scaled and unsuitable for measurement of humidity in narrow spaces.
Thus, humidity sensing devices and apparatus now on the market all have certain drawbacks in their functional characteristics, resistance to environmental conditions, price, stability over a long period of time and/or convenience for usage and maintenance: none of them is fully satisfactory in every respect.
U.S. patent application Ser. No. 912,714 filed June 5, 1978, by Nishino et al, now U.S. Pat. No. 4,217,623, discloses a humidity sensing element of an electrostatic capacitance change type, comprising a substrate of a valve metal, a dielectric oxide layer formed by anodization of a surface region of the substrate and a gas permeable counter-electrode layer formed on the dielectric layer preferably using a semiconductive metal oxide such as manganese dioxide as an inner half of the counter-electrode layer, on the condition that, microscopically, the counter-electrode layer is only partially in contact with the dielectric layer.